Various methods of optically measuring the particle size of particles in a dispersion medium have been suggested. For example, in a process of producing nano-sized particles used for a resist or the like of a semiconductor, that is, in a process of synthesizing a so-called “nano material”, the particle size of obtained particles is measured in order to control the quality of the particles.
For example, Patent Document 1 discloses a batch type method of measuring particle size distribution of nano-sized particles through sampling. First, a dispersion liquid is obtained by dispersing a particle group with known particle size distribution in a dispersion medium used for measurement, and then the dispersion liquid is allowed to stand. Further, the temporal change in the number of photons is measured by irradiating the dispersion liquid with laser light and detecting scattered light from the particle group using light detecting means. The mobility and the fluctuation rate of the particle group are obtained by data processing means based on the Brownian motion of the particle group to be measured and the force applied to the particles is calculated to obtain the correlation between the fluid shear stress and the fluid shear rate of the dispersion medium on the surface of the particles. Next, the particle size distribution of a particle group whose particle size distribution is unknown is determined in order to satisfy the correlation between the fluid shear stress and the fluid shear rate which have been previously acquired in the same manner as described above.
Batch type measurement through sampling has been performed as the method disclosed in Patent Document 1, but it is desired that the particle size of particles in a flowing dispersion medium is able to be simply measured by not performing an in situ measurement but an in-line measurement because a measurement time is also required. Further, a particle counter has been mainly used in in-line particle size measurement, and a so-called particle size in terms of polystyrene latex calibrated with a standard substance such as polystyrene latex is used in the particle counter. However, it is desired that the particle size to be measured is not the particle size in terms of polystyrene latex and standard substance calibration is not required. A device that performs measurement on particles having a particle size larger than the nano size has been suggested as such an in-line particle size measuring device.
For example, Patent Document 2 discloses a particle size measuring device which measures the particle size of particles of the order of micron in a dispersion medium flowing in a channel at a predetermined speed. In such a device, particles in a dispersion medium are photographed and the particle size thereof is measured from the obtained image data. Particularly, since the apparent particle size varies depending on the difference in photographing distance, the apparent particle size is corrected by obtaining the flow rate of measured dispersion medium, a position in a direction perpendicular to a photographing direction and a flow direction, and the speed of particles in the flow direction from the image data, and calculating the position of particles in the photographing direction based on the Navier-Stokes equation.
Further, Patent Document 3 discloses a particle size measuring device which performs measurement by using focused light beams focused at a focal point with a condensing lens in measurement of the particle size of particles of the order of submicron in a dispersion liquid flowing in a channel at a predetermined speed. Since the focused light beams applied to the dispersion liquid are optically changed by the particles, the change is measured to acquire the particle size correlated with the change. Further, since the optical change varies depending on a distance from a light source to a passing position of the particles, the optical change is corrected by the passing position of the particles. The passing position of the particles is able to be calculated using the time taken for the particles to pass through the focused light spreading from the focal point.